Caption: Aurora borealis or northern lights, from space. Aurorae (green, right) are caused by energetic charged particles from the Sun colliding with gas molecules in the atmosphere near the poles. The lights across bottom are those of unidentified cities, which are blurred due to the long exposure. Also seen is airglow (green, across centre), a photochemical luminescence in the upper atmosphere. In this case the colour is due to oxygen atoms. Although always present above the Earth, airglow is only seen at night by looking edge on from space. Photographed by the crew of the Space Shuttle Endeavour during Mission STS-108 of 5-7 December 2001.